1. Field of the Invention
The present invention relates to optical communications, and more particularly, to an optical transceiver module for optical communications.
2. Description of the Related Art
An optical transceiver includes a light source, a light source driver, a light receiver, a light filter, a preamplifier, etc. The optical transceiver is usually formed by hybrid integration of these parts. Thus, many attempts have been made to integrate the light source, the light filter, and the light receiver on a single substrate. Additionally, many attempts have been made to integrate only the light source and the light receiver on a single substrate. When the parts are integrated on a single substrate, a package cost can be reduced due to simple packaging. In the optical transceiver module, the package cost is more than half of the total cost of the parts. Thus, it is necessary and important to reduce the package cost by using single-integrated optical devices.
Accordingly, many attempts have been made to form the light source and the light receiver on a single substrate.
FIG. 1 is a schematic view illustrating an optical transceiver unit in which a laser diode (LD) and a photo-detector (PD) are integrated in an in-line manner into a single substrate. That is, the optical transceiver unit is a device in which a light source and a light receiver are integrated into the single substrate in an in-line manner. A portion L indicates a light source portion corresponding to the LD. A portion D indicates a light receiver portion corresponding to the PD. On the other hand, a portion A indicates an absorber between the LD and the PD, which prevents light emitted from the LD from being delivered to the PD. Waveguides or active layers 1, 2 and 3 in the LD, the absorber, and the PD are formed on the substrate along the same axis. Moreover, input light and output light are delivered through the waveguides 1, 2 and 3.
Feedback is achieved through a diffraction grating G. The diffraction grating G and the active layer 1 supplying a gain make up a distributed feedback (DFB) LD that is used as the light source. Accordingly, the LD outputs an output signal S1. Since an input signal S2 uses a transparent wave with respect to the waveguide 1 of the LD and the waveguide 2 of the absorber, the input signal S2 is not absorbed into the LCD and the absorber, and delivered to the PD. For example, a wavelength of the output signal S1 is 1.3 μm, and a wavelength of the input signal S2 is 1.55 μm. Moreover, the active layer 1 of the LD and the active layer 2 of the absorber have a bandgap wavelength of 1.3 μm, and the active layer 3 of the PD has a bandgap wavelength of 1.55 μm. Accordingly, although the input signal S2 passes through the active layer 1 of the LD and the active layer 2 of the absorber, and then reaches the PD, the output signal S1 cannot reach the PD. Other contents related to the optical transceiver unit are disclosed in OFC98, p. 350.
The single-substrate integrated optical transceiver unit disclosed above has the disadvantage of generating electrical crosstalk.
FIG. 2 illustrates a structure of an optical transceiver module using the optical transceiver unit of FIG. 1, which includes a module structure driving the LD and reading a signal input to the PD in the single-substrate integrated optical transceiver unit. The detail description related to FIG. 2 is disclosed in U.S. Pat. No. 6,148,015.
Referring to FIG. 2, a method of driving the LD is as follows.
An optical current generated in the absorber A, which prevents light emitted from the LD from being absorbed into the PD, is read and delivered to a controller 16. The controller 16 compares the optical current to an appropriate physical quantity (particularly, a voltage V1), and then supplies a direct current (DC) into the LD. The controller 16 is a voltage comparator comparing voltages at both terminals and outputting a comparison result. At this point, the absorber A serves as a monitor PD in a conventional optical transceiver unit, and the method of driving the LD by controlling the DC drive current of the LD using the read current of the monitor PD is one of the conventional methods. An inductor 14 is placed in front of the controller 16, and a capacitor CI placed in front of an input terminal LI delivers an alternating current (AC) that drives the LD.
Next, a method of reading a signal input to the PD is as follows. The PD is operated by supplying a reverse bias voltage. That is, the reverse bias voltage is applied to the PD by applying a ground voltage to a substrate 10 and applying a negative voltage from a negative power source 19. An inductor 17 in front of the negative power source 19 allows only a DC current to pass through. Meanwhile, a capacitor CO transmits a detected AC component to an output terminal DO.
In this method, the substrate of the single-substrate integrated optical transceiver unit is set as the ground, a plus bias voltage is applied to the LD, and also a negative bias voltage is applied to the PD. The disadvantage of the method is that the positive AC power applied to the LD is partially delivered to the PD connected to the negative power source 19.
FIG. 3 is a circuit diagram of the optical transceiver module of FIG. 2.
Referring to FIG. 3, a light source L, an absorber A, and a light receiver D are diodes. Here, the light receiver D, which is also a diode, receives a reverse bias voltage. The light receiver D, to which the reverse bias voltage is applied, serves as a capacitor when the AC is supplied. Accordingly, an AC power input to the light source L is partially delivered to a node 1 of a negative voltage through the diode D to which the reverse bias voltage is applied. This phenomenon is called electrical crosstalk that reduces the sensitivity of a receiver drastically.
When noise of −60 to −70 dBm is input to an input terminal of a preamplifier of the light receiver or the receiver, the receiver generates an error. An electrical signal delivered to the light source has a power larger than +10 dBm. Consequently, when more than −70 to −80 dB of a signal applied to the light source is delivered to the receiver, the receiver generates an error. Especially, this problem is more serious in manufacturing an optical transceiver using a single-substrate integrated optical transceiver unit than in hybrid-integration of a light source and a receiver.